1. Field of the Invention
The present invention relates to an optical-type rotational body position detection apparatus.
2. Description of the Related Art
An optical-type rotational body position detection apparatus is already known from, for example, Japanese Patent Application KOKAI Publication No. 2000-162335.
This publication discloses an apparatus for optically detecting standard positions of a second wheel, a minute wheel and an hour wheel in order to detect standard positions of a second hand, a minute hand and an hour hand in a hand type wrist watch, that is, an optical-type rotational body position detection apparatus.
The second wheel of the hand type wrist watch is rotated in a unit of a predetermined angle at each of predetermined time intervals by a first driving system which includes a first driving motor and a plurality of intermediate wheels. Each of the minute wheel and the hour wheel of the hand type wrist watch is rotated in a unit of a predetermined angle at each of predetermined time intervals by a second driving system which includes a second driving motor and a plurality of intermediate wheels. The second wheel, the minute wheel and the hour wheel are coaxially rotatably supported by a main plate. The optical-type rotational body position detection apparatus includes a light emitting device functioning as a light emitting unit, and a light receiving device functioning as a eight detecting unit, which are disposed in both sides of the second wheel, minute wheel and hour wheel.
A standard position light transmission hole is formed in each of the second wheel, minute wheel and hour wheel so that the light transmission hole is positioned on a rotational locus on each wheel, which intersects an optical axis connecting the light emitting device and the light receiving device.
With the above-described conventional optical-type rotational body position detection apparatus which is combined with the conventional hand-type wrist watch, when a need has occurred to set the standard positions of the second hand, minute hand and hour hand (i.e. the standard positions of the second wheel, minute wheel and hour wheel), the second wheel is rotated by the first driving system and the minute wheel and hour wheel are rotated by the second driving system while light is being emitted by the light emitting device. When the standard position light transmission holes of the second wheel, minute wheel and hour wheel coincide with the above-described optical axis during the rotations of these wheels, the light receiving device can detect the light from the light emitting device and it is recognized that at this time the second hand, minute hand and hour hand (i.e. the second wheel, minute wheel and hour wheel) are set at their standard positions.
However, owing to the manufacturing tolerable error of each of the second wheel, minute wheel and hour wheel, the manufacturing tolerable error of each of the plural intermediate wheels of the first driving system and the manufacturing tolerable error of each of the plural intermediate wheels of the second driving system, back-lashes occur in each of the combination of the second wheel and the various intermediate wheels of the first driving system and the combination of the minute wheel, and hour wheel and the various intermediate wheels of the second driving system. In addition, the standard position light transmission hole of each of the second wheel, minute wheel and hour wheel has a manufacturing tolerable error.
Thus, even if the standard position light transmission hole of each of the second wheel, minute wheel and hour wheel coincides with the above-described optical axis and the light receiving device can detect the light from the light emitting device so that it is detected that the second wheel, minute wheel and hour wheel are positioned at their standard positions (i.e. the second hand, minute hand and hour hand are positioned at their standard positions), it is possible that the center of the standard position light transmission hole of each of the second wheel, minute wheel and hour wheel does not coincide with the above-described optical axis and hence the second hand, minute hand and hour hand are slightly displaced from their standard positions. This means that, in the case where the standard positions of the hour hand, minute hand and second hand are at 0 hour: 0 minute: and 0 second, the second hand, minute hand and hour hand do not completely overlap with each other at these standard positions.
The present invention has been made in consideration of the above-described circumstances, and an object of the present invention is to provide an optical-type rotational body position detection apparatus which can detect the position of a rotational body more precisely than in the prior art.